1. Field of the Invention
The present invention relates generally to methods and devices for mounting a sensor securely to variably shaped surfaces. One specific embodiment of the invention relates to mounting a biomass sensor to the external surface of a biofermenter.
2. Description of Prior Art
i. Mounting Methods for Non-Invasive Sensors
Non-invasive sensors are widely used in the analysis of material properties. A great advantage of such sensors is that they allow measurements to be made without consuming, infecting, or damaging the material being analyzed. However, for such sensors, maintaining a fixed geometric relationship between the sensor and the material under test is frequently essential to maximizing the accuracy and reproducibility of the measurement. Specifically what is frequently required is to maintain a fixed distance and angle between the sensing components in the sensor, and the surface of the material or the surface of the container holding the material. A particular challenge for sensor mounting is posed when the container holding the material or the material itself is cylindrical in shape, and the radius of the cylinder may vary widely. Prior art has addressed this subject in a variety of ways.
U.S. Pat. No. RE36130—Haynes discloses a method for measuring pipe thickness using a sensor attached to a glide plate that maintains contact with the pipe. The glide plate has a curved underside that matches the shape of the outer surface of the pipe. A manual means of adapting the apparatus for different pipe diameters is provided, but only for a limited range of radii. In addition, the distance between the individual sensor components and the pipe will vary with pipe diameter unless a new glide plate is used for each new pipe diameter.
U.S. Pat. No. 6,622,561—Lam discloses a method of detecting flaws in tubes, employing a sensor mounting pad in one of several possible configurations: (1) the surface of the mounting pad is designed so that it corresponds in shape to the tube, (2) the mounting pad is fabricated from flexible material, or (3) an inflatable component is adjusted to conform the mounting pad to the shape of the tube. The disadvantage of these methods is that they are not automatically adaptable to variable radius surfaces (ie. they requires changing the mounting pad, raising or lowering shoe, or changing the bladder inflation). In addition, this method only provides a means of maintaining a fixed distance between the tube and the entire sensor, as opposed to the individual components in the sensor. As the tube radius is changed, even if a fixed distance is maintained between the tube and the nearest point on the sensor body, the distance between the individual sensor components and the pipe body may change as a result of the changing curvature of the tube.
U.S. Pat. No. 5,007,291—Walters discloses a method of mounting a sensor in close proximity to a pipe in order to inspect its quality. A proximity sensor is used in conjunction with feed-back control and a hydraulic mechanism to maintain a fixed distance between the sensor and the surface of a rotating pipe under inspection. This method has the disadvantage of being cumbersome, complex, expensive, and automatically adaptable to only a narrow range of pipe radii.
U.S. Pat. No. 4,320,659—Lynnworth discloses a system for measuring fluid impedance or liquid level through the wall of a cylindrical container. A method for mounting the sensor is described that maintains floating contact between the sensor and the external wall of the container. The sensor mount is held in place by either magnetic or adhesive means. The methods provided are not automatically adapting to varying radius containers. In addition, the magnetic attachment method provided will only work on steel containers and may be insufficiently secure in high vibration environments. Adhesive attachment is more secure and applicable to a wider array of surfaces, but is not easily removable. Adhesives typically require surface preparation and may fail under constant load. In addition, the use of adhesive would hinder rapid transfer of the sensor between containers, would limit its reusability, and may foul the surface of the vessel being probed.
U.S. Pat. No. 5,644,093—Wright discloses a sensor mounting method employing adhesive pivoting feet that are adaptable to variable radius cylindrical surfaces. In addition, a spring is provided for maintaining contact between the sensor and the mounting surface. The disadvantages associated with the use of adhesive for attachment are described above. The use of pivoting feet reduces the strength of the mounting device and increases the likelihood that the mount could flex or fail in response to force, vibration, or temperature changes. Also, the position of the sensing components within the sensor is not automatically fixed with respect to the mounting surface. Therefore, variation of the radius of curvature of the mounting surface may result in variable distances and angles of the sensor components relative to the surface.
U.S. Pat. No. 4,019,373—Freeman discloses an ultrasonic transducer for sensing flow velocity through a pipe wall. A U-shaped member is used to provide stable mounting of the transducer onto the pipe. Transducer position is adjusted axially by means of a screw thread. The disadvantage of this method is that only a narrow range of pipe radii may be accommodated. In addition, manual adjustment of transducer position is required in order to achieve a fixed distance between the transducer and pipe surface. As a result, the device is not rapidly transferable between pipes, and may be prone to user error, if the transducer position is not carefully adjusted.
U.S. Pat. No. 4,242,744—Rottmar discloses a method of fixing a sonic or ultrasonic transducer to a container for level detection. A spring is used to urge the transducer towards the container. The disadvantage of this method is that no means is provided to maintain a fixed angle between the sensor and the container. As a result, a fixed geometry between the transducer components and container may not be maintained between different placements of the transducer either on the same container or on containers of different size.
ii. Measurement of Biomass in Liquid Cultures
Liquid cultures of cells or microorganisms are frequently grown for research purposes or for commercial gain. Cells or microorganisms can be genetically modified to produce high yields of chemicals that may be difficult, expensive, or impossible to synthesize by other means. In order to prevent growth of other undesirable cells or microorganisms in the same liquid culture, it is important that the culture be grown under sterile conditions. For this reason, the growth medium is sterilized prior to inoculation with the desired cell or microorganism. In order to maintain a barrier to foreign organisms and optimize the growth of the desired cell or microorganism, liquid cultures are frequently grown under highly controlled conditions in what are referred to as fermenters or bioreactors. For research purposes, fermenter vessels are typically cylinders constructed from glass or plastic, having liquid capacities ranging from less than 1 L up to 20 L. For larger scale production, stainless steel tanks with capacities from 10 L up to thousands of liters or more are frequently used. A flat glass port is typically provided on the side of such vessels, to allow for viewing of the liquid culture. In addition to maintaining sterile conditions, fermenters may provide control over such parameters as temperature, pH, rate of stirring, and concentration of nutrients and dissolved gases.
Cells or microorganisms typically undergo several stages of growth in a fermenter. After inoculation, the initial growth rate of the cells or microorganisms may be slow, as the organism becomes accustomed to its new environment. This is frequently followed by a rapid growth phase where the biomass increases nearly exponentially. This growth period is sometimes referred to as the “log phase” due to the fact that the change in the logarithm of biomass is nearly linear with time. Eventually, as the nutrient supply relative to the biomass diminishes, the growth will slow. In order to achieve maximum biomass, the conditions in the fermenter need to be changed during the different phases of growth. Ideally a feedback mechanism would link the measured growth of the cells or microorganisms to the conditions in the fermenter. Frequently, a physical or chemical stimulus is used to induce production of a desired chemical by the cells or microorganisms. The timing of this induction relative to the growth cycle of the cells or microorganisms is often critical in order to achieve maximum chemical yield. Unfortunately, methods of continuously and reliably measuring the growth of cells or microorganisms in liquid cultures are not widely available.
The most commonly used method of measuring the biomass in liquid cultures is by extracting a portion of the liquid and measuring its optical density in a spectrophotometer. This method has several disadvantages: (1) each time liquid is withdrawn, there is a risk that the culture will be contaminated, (2) the method is not continuous, and (3) the method is labor intensive, requiring frequent extraction and precise volumetric dilution of the extracted liquid when high cell concentrations are measured. Commercial devices are available (eg. Wedgewood Technology, Incorporated, Model 650 “Absorbance Monitor”) that offer continuous measurement of optical density using a probe that is immersed in the liquid culture. Unfortunately, such devices are prone to drift, particularly due to growth of cells or microorganisms on the sensor itself.
Non-invasive methods of biomass monitoring have also been described in prior art. U.S. Pat. Nos. 5,483,080—Tam and 6,573,991—Debreczeny both describe non-invasive reflectance sensors for measuring biomass in liquid cultures through the wall of a fermenter. By these methods, biomass is measured free from the risk of contaminating the culture. However, sensor mounting methods are not provided that automatically compensate for changes in the shape and size of the fermenter. Due to the wide variety of sizes and shapes of research and production-scale fermenters, external mounting of sensors to the fermenter presents a particular challenge.